Cerium divalent complexes

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Pyrazolin-5-ones form complexes with both inorganic and organic compounds much more readily than do the 2-pyrazolin-5-ones. The most extensive series of complexes is that formed with a variety of metallic salts. Antipyrine (2,3-dimethyl-l-phenyl-3-pyrazolin-5-one) forms a series of complexes with salts of divalent, trivalent and tetra-valent metals. Two molecules of antipyrine form a complex with one molecule of copper, cadmium, cobalt and zinc salts.266,866,1116 Complexes prepared from metallic nitrates are usually hydrated.1322 There also exists a series of complexes in which three molecules of antipyrine form a complex with one or two molecules of metallic salts. Such complexes form with two molecules of simple ferric salts272 or with one of complex iron cyanides.608 Nitrates of thorium, lanthanum, cerium and samarium also give such complexes.841 This ratio also occurs in some antipyrine complexes with cadmium and zinc thiocyanate.266 A number of salts of rare earths and iron which have complex anions such as thiosulfate, thiocyanate, dithionic acid and complex iron cyanides form complexes in which six molecules of antipyrine are present.405,408 608,841,950 Stannic chloride forms salts containing three or four molecules of antipyrine and hydrochloric acid.46... 

Furthermore, irt the elements Ce to Yb, f-orbitals come into play, to such an extent that most zero-valent metals in this series (except Ce and Gd) have the configuration [Xe]4f 5d°6s, which opens the possibility of finding divalent compoimds in which the dipositive ions have the [Xe]4f"5d°6s° configuration instead of [Xe]4f 5d 6s°. This former configuration, as we shall see later, is actually present in many divalent molecular complexes. In the special case of cerium, the + IV oxidation state with configuration [Xe]4f°5d°6s° is well documented, and we will not consider the Ce molecular compoimds in this survey. 

From this criterion one finds that the tendency towards inner-sphere complexity is greater for indium than for cerium and that for the divalent transition metals (0,3 M MSO solutions) it increases in the order Ni < Mn < Co < Zn < Cu < Cd. 

As the data in table 32.7 indicate, several of the R-X equilibrium diagrams are very complex. Thermal analysis data, cf. table 32.3, show that in general the intermediate phases melt incongruently. The Tm-Cl system (Caro and Corbett, 1969), is obviously the most complex of the known systems. In addition to the binary phases listed in table 32.7, several mixed-metal phases have also been described. These include the face centered cubic (Nd, Ce)Cl2.2o and the (Nd,Ce)Cl2, 37 compositions in which divalent cerium is observed (Druding and Corbett, 1961). The PrCl2.37 phase, which decomposes below 594°C, is stabilized at low temperatures by substitution of Nd (Druding et al., 1963). 

Charge transfer bands result whenever an easily oxidized ligand is bound to a trivalent lanthanide ion which can be reduced to the divalent state or when the ligand is bound to one of the tetravalent ions (J0rgensen, 1970). Such transitions are commonly observed in the spectra of complexes of samarium(III), euro-pium(III), thulium(III), ytterbium(III), and cerium(IV). The position of these bands in the spectrum is markedly dependent on the ligand and the metal ion. For example, in the ions RCU the charge transfer bands for europium(III),... 

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